Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $221.63   13.586%   $251.7407   $251.74   $251.75   $251.70  
  Purchase #2 $70.78   11.527%   $78.9388   $78.94   $78.95   $78.90  
  Purchase #3 $148.45   4.196%   $154.679   $154.68   $154.70   $154.70  
  Purchase #4 $213.58   13.387%   $242.172   $242.17   $242.15   $242.20  
  Purchase #5 $16.87   5.357%   $17.7737   $17.77   $17.75   $17.80  
  Purchase #6 $86.29   13.174%   $97.6578   $97.66   $97.65   $97.70  
  Purchase #7 $216.93   13.575%   $246.3782   $246.38   $246.40   $246.40  
  7 purchase totals = $974.53       $1,089.3402   $1,089.34   $1,089.35   $1,089.40